A Lethal Hazard for Birds

Clear and Reflective Windows: An Invisible and Lethal Hazard for Birds

by Daniel Klem, Jr. Ph. D.
July 2008

Sheet glass and plastic in the form of windows—from tiny garage panes to entire walls of multistory commercial buildings—are killing wild birds the world over. Because of the uniqueness and limitations of life, we humans cannot be certain how birds see the world, but they clearly behave as if clear and reflective windows are invisible to them. The consequence is that a bird striking a window after leaving a perch from a bit over three feet away can seriously injure or kill itself outright, not from what many assume is a broken neck, but from head trauma and related injuries that likewise can kill us in comparable accidents.

Like humans who sustain a powerful blow to the head in a car accident or by other means, the brain swells, breaking the blood-brain barrier; the victim struggles to breathe, lapses into a coma, and at least for birds, one out of every two strikes results in a fatality. Casualties are typically injured and vulnerable to predators and scavengers that quickly take the dead and dying from beneath the offending window, on whose surface the reflected vegetation and sky were the goal of the victim. These collisions are universal, occurring wherever birds and windows coexist in urban, suburban, and rural habitat worldwide. Billions of birds are killed annually at the attractive and utilitarian windows of our homes and workplaces, and extensive evidence documents that this source of avian mortality kills more birds than any other human-associated factor except habitat destruction.

Habitat destruction is our greatest assault on wildlife because when we markedly alter or eliminate a habitat we also eliminate the fundamental resources upon which life depends; life cannot endure without food, shelter, or water. Certainly windows kill far more birds than wind turbines, power lines, pesticides, and domestic cats. Yet it is fair to claim that this horrific loss of birdlife from collisions with windows is largely ignored by most people, and most regrettably by those who can do the most to prevent it: members of our conservation community, whose mission is to protect life, and the building industry, whose mission is to construct human structures, increasingly with modern designs lavishly covered with glass.

Why is the glass threat to birds largely ignored? After studying bird-glass collisions for over 34 years, my view is that the attractive and utilitarian value of windows for humans is so great—economically, psychologically, and aesthetically—that we cannot imagine such a valuable product doing harm, or even consider altering it in any way that would inhibit the meaningful benefits it offers. I have never advocated eliminating windows in human structures, but I have recommended covering or altering panes for those willing to make a small change in the way they see through their windows in order to protect birds. Ideally, I have tried to investigate how new types of glass can be manufactured such that we humans see through windows the way we do now, but birds viewing the same windows from the outside see patterns in the panes that alert them to danger.

Covering windows with conventional insect screening is an effective lifesaver because it keeps a flying bird from striking the unyielding surface. Many homeowners have built their own safety nets, and a commercial source has existed for some time to purchase residential home screening. Covering reflective windows on the outside surface with patterns, such that the elements making up the pattern are two to four inches apart, transforms these panes into barriers that birds avoid. If the pattern is oriented vertically like blinds, the elements making up the columns can be as much as four inches apart, but if oriented horizontally in rows the elements need to be two inches apart. The actual elements of the overall pattern can be any opaque or translucent object: circles, diamonds, stripes, hawk silhouettes, spider webs, or ultraviolet-reflecting maple leaves. My experiments reveal that if you cover an offending window with patterns composed of elements with this density, you eliminate bird strikes altogether. Applying patterns with greater spacing between elements reduces but does not eliminate fatal strikes. The more elements, the greater the protection and the more lives saved; fewer elements mean greater risk and more lives lost.

My most recent experiments have produced some hopeful results for using ultraviolet patterns to alert birds to the dangers of windows. But whether ultraviolet signals, or some other inventive techniques, are actually effective in making glass and plastic safe for birds, manufacturers must be convinced that there is a market for such products. It is therefore imperative that we work together to educate our friends in conservation and those in the building industry that saving birds from this unintended lethal hazard is a practical, ethical, and moral commitment we must make to one of nature’s most useful and exquisite creations.

I know no one purposefully seeks to kill birds at the windows of their homes or workplaces. There are international treaties that forbid the intentional killing of even a single bird, yet so complicated and pervasive is the glass threat that only recently have federal agencies begun to acknowledge the need to address the toll that glass is exacting on protected birds.

The U.S. Green Building Council has created an evaluation system called Leadership in Energy and Environmental Design (LEED) to encourage the construction of environmentally responsible structures, so-called “green buildings.” But none of these sanctioned and award-winning structures are green to me if birds are killed striking the energy-saving windows bathing interiors with natural light, which in turn frequently nurture internal plantings that lure nearby birds to a crashing death from an invisible obstacle. Those charged with creating new versions of the LEED evaluation system need to be educated and encouraged to include a process for designing bird-safe glass and surrounding landscape architecture.

To this end, the New York City Audubon Society, the City of Toronto, and groups in Chicago have recently published bird-safe building guidelines. These recommendations are fundamental to educating architects so that their creative designs will be safe and ensure the survival of birds, among them common species as well as rare, threatened, and endangered ones. Winning the hearts and minds of architects in this worthy cause of stopping the unintentional killing of birds is essential if glass manufacturers are to judge there is a market worthy of their investment in creating bird-safe glass. No one wants this unintended killing to continue. Surely this extravagant source of bird mortality must be curbed or stopped completely if future generations are to appreciate, use, and enjoy these magnificent life forms before their diversity and numbers become too rare to see.